NATO said there’s a risk of Russia sending troops into Ukraine under the “pretext” of a humanitarian or peacekeeping mission after President Vladimir Putin massed soldiers on his country’s western border.

“We’re not going to guess what’s on Russia’s mind, but we can see what Russia is doing on the ground — and that is of great concern,” Oana Lungescu, a spokeswoman for the North Atlantic Treaty Organization, said by e-mail today. “The latest Russian military buildup further escalates the situation and undermines efforts aimed at finding a diplomatic solution to the crisis. This is a dangerous situation.”

The standoff over Ukraine is escalating in the wake of the U.S. and the European Union hitting Russia with the toughest sanctions yet, with Putin ordering economic retaliation. In Ukraine’s easternmost regions, government troops are pressing ahead with an offensive against pro-Russian insurgents that’s prompted the government in Kiev to say yesterday that victory was near.

Warnings of the threat of invasion are intensifying after reports that Russia has massed forces on its neighbor’s border in the biggest military buildup since troops were withdrawn from the area in May.

“Putin will likely aim at a fig leaf policy with an incursion under the guise of a peacekeeping operation,” Amanda Paul, an analyst at the European Policy Centre in Brussels, wrote in an e-mail. “As other conflicts in the former Soviet Union have shown, once there is a Russian military presence it usually never leaves.”

The escalation is also rattling global markets. U.S. stock- index futures declined, after the index closed at a two-month low. Standard & Poor’s 500 Index futures expiring next month fell 0.4 percent to 1,906.3 at 1:55 p.m. in London, erasing earlier gains of as much as 0.3 percent. The Stoxx Europe 600 Index dropped 1.3 percent to 327.32.

Poland, a NATO member that borders Ukraine and has been among the EU nations seeking the toughest response to the Kremlin’s policy over Ukraine, earlier today said the risk of a Russian invasion has increased.

Poland has “reason to believe” that the risk of an incursion is “greater than a few days ago,” Prime Minister Donald Tusk told reporters in Warsaw today. His foreign minister, Radoslaw Sikorski, yesterday said that any incursion would be under the guise of a peacekeeping operation. Russia yesterday called for a humanitarian mission to eastern Ukraine, which is on the verge of a “catastrophe,” the Foreign Ministry in Moscow said on its website.

Ukraine considers Russia’s proposal of humanitarian aid to be “cynical” and blames its neighbor for not allowing the military conflict to end, Andriy Lysenko, a spokesman for the country’s defense and security council, said at a news conference in Kiev today.

Ukraine would treat Russia dispatching peacekeepers as “direct aggression,” Hennadiy Zubko, the first deputy chief of President Petro Poroshenko’s administration, told reporters in Kiev today. He described the conflict as a “war between Russian special forces, paid Russian mercenaries and Ukraine.”

The government in Kiev has estimated that Russia has deployed 45,000 soldiers, 160 tanks and 192 warplanes among other equipment, including soldiers stationed in Crimea.

Even so, there’s reason to be skeptical about Russia’s willingness to go into a military conflict, according to Stefan Meister, an analyst at the German Council on Foreign Relations in Berlin.

“I cannot believe Russia would attack Ukraine, which would worsen the situation further and deepen the conflict with the West,” Meister said by phone today. “I don’t see signs for invasion. In fact, Russia has tried to bring in some distance from the separatists.”

Ukraine’s forces were attacked by Russian troops over the border, taking shellfire for four hours yesterday in the Luhansk region, Lysenko said. Eighteen Ukrainian soldiers were killed and 54 wounded in the last 24 hours, during which time they engaged the rebels in 25 firefights, Lysenko said.

Three civilians were killed amid overnight shelling in the city of 1 million, the artillery and mortar fire also damaging infrastructure and residential buildings, the Donetsk city council said on its website today.

Russia’s Investigative Committee today said Ukraine deployed prohibited weapons against civilians, citing “irrefutable evidence” of the use of H-17 liquid flame explosives near the city of Slovyansk. Russia plans to prosecute those responsible and may share its findings with international courts, according to a website statement.

Aleksey Dmitrashkovsky, a spokesman for the Ukrainian military, said that the army didn’t attack civilians and didn’t use prohibited weapons.

The humanitarian situation in Ukraine is steadily worsening, John Ging, director of humanitarian operations for the United Nations, said at an emergency meeting of the Security Council yesterday in New York. He said the fighting has killed at least 1,367 people — both civilians and combatants — and wounded 4,087 since mid-April. Ging cited a Russian estimate that 740,000 Ukrainians have crossed into Russia since the beginning of the year.

While Russia has repeatedly denied any involvement in the conflict, the U.S. and its EU allies say Putin is supplying the insurgents with weapons, manpower and financing and say he could stop the war if he reined in the separatists.

Russian Ambassador to the UN Vitaly Churkin told the Security Council that the situation in Ukraine, particularly in Donetsk and Luhansk, is “disastrous” and called on “the international community to take emergency measures to improve the humanitarian measures” in the area.

The situation in Luhansk, a city of more than 400,000, remains “critical” as residents have no access to electricity and water, while both fixed-line and mobile phone connections are off, the local council said on its website. Shops are closed, limiting the availability of food and fuel, it said.

The pro-Russian separatist forces, which now number about 15,000, up from 300 when the conflict started, hold less than half the territory they did four weeks ago, Ukrainian Defense Minister Valeriy Geletey said in a BBC interview broadcast this week.

Meanwhile, an international team continued to search the crash site of Malaysia Airlines Flight MH17, which the U.S. says was probably shot down last month with a missile fired by the insurgents. The rebels and Putin’s government blame Ukraine.

The investigators determined that parts of the site have been searched before their arrival, Pieter-Jaap Aalbersberg, head of the Dutch recovery mission, told reporters in Kiev today. He urged people living in the area to bring any personal belonging they collected to the local playground.

“It appears that local residents and emergency services conducted research and recovered victims’ bodies and personal belongings,” Aalbersberg said. “As far as experts can judge at this time, they did it in a careful and orderly fashion. We are very grateful for the help and respectful approach.”

NATO said there’s a risk of Russia sending troops into Ukraine under the “pretext” of a humanitarian or peacekeeping mission after President Vladimir Putin massed soldiers on his country’s western border.